Canisters differ from motor vehicle tanks above all in that in normal use they are tightly sealed. In other words, they have no ventilation. A consequence of this is that often considerable internal pressure is created in the closed canister, which constitutes a substantial loading on the canister and its closure means. In addition, canisters must be able to withstand certain test loadings. For example, they must remain stationary on a flat surface which is inclined at 20.degree.. They must be able to withstand being dropped from a height of 10 meters without damage and only a quite specific and extremely small quantity of the contents may be lost within a given time. Materials used hitherto have been sufficiently tight where they have to hold gasoline (petrol) which contains lead. There is, however, an increasing endeavour to achieve lead-free petrol which then instead of the lead contains other additives, for example aromatic substances, acyclic and alicyclic compounds. The materials long since used for plastic canisters suffer however from considerable permeation, particularly in respect of these additives, so that they cannot comply with legal requirements.
Methods have therefore been developed whereby canisters consisting of standard polethylene are subsequently provided with a barrier layer by the sulphonation process or fluorination process. This coating is however very thin and correspondingly the barrier action is not very great. In the most favourable case, a permeation reduction of 25% is achieved. This value furthermore relates to motor vehicle tanks in which, of course, there is no internal pressure. Because the internal pressure in canisters accelerates permeation, these methods are not adequate for the particular field of application which involves canisters.